1. Field of the Invention
The present invention relates to a redundant array of inexpensive disks (RAID) controller for controlling multiple storage-devices constituting a RAID array.
2. Description of the Related Art
As the capacity of hard disks is increasing rapidly, the extent of damage caused by hard disk failures is increasing. RAID technology is used to prevent data loss caused by hard disk failures and to improve processing performance.
RAID technology provides techniques for combining multiple hard disks and managing them as a redundant single hard disk. The techniques are categorized into six levels RAID 0 through 5 according to the method of data arrangement on the hard disks and the method of providing data redundancy.
For example, RAID 1 allows data to be simultaneously written to two or more hard disks. Therefore, RAID 1 is also referred to as “mirroring”.
If one hard disk fails, RAID 1 allows automatic switching from the failed hard disk to another hard disk on which data can be processed, and thus enables the ongoing operation to continue. RAID 1 cannot improve access speed, but has advantages that it can prevent data loss and system failures caused by disk failures.
If a hard disk in a RAID 1 array fails and needs to be replaced, data stored on this failed hard disk has to be copied to a backup hard disk.
This involves reading of data stored in all areas of a properly functioning hard disk in the RAID 1 array, and writing of the read data to the backup hard disk.
If access from a high-level device (e.g., server) continues during this data transfer, the access from the server and the data transfer are carried out in parallel with each other. This causes degradation in the access performance of the server.
Moreover, since the amount of time required for data transfer associated with the replacement of a hard disk increases in proportion to the capacity of the hard disk, the estimated period of time during which the access performance of the server is degraded also increases in proportion to the capacity of the hard disk.